Chapter 2. Microstructural Evolution in Thin Films
Chapter 2. Microstructural Evolution in Thin Films of
Electronic Materials
Academic and Research Staff
Professor Carl V. Thompson, Professor Henry i. Smith, Dr. Paul Evans, Dr. En Ma, Dr. John Melngailis, Dr.
H. Miura
Visiting Scientists and Research Affiliates
David J. Edell,' Harold J. Frost, 2 David A. Smith, 3 King-N. Tu 3
Graduate Students
Jaeshin Cho, S. Cooperman, Andrew D. Dubner, Jerrold A. Floro, H. Inglefield, Y-C. Joo, Harold Kahn,
Yachin Liu, Hai P. Longworth, Jaesang Ro
Technical and Support Staff
Celia Slattery
2.1 Coarsening of Particles on
a Planar Substrate
2.2 Epitaxial Grain Growth
Sponsor
National Science Foundation
National Science Foundation
U.S. Air Force - Office of Scientific Research
Project Staff
Project Staff
Professor Carl V. Thompson, Yachin Liu
Jerrold A. Floro, H. Inglefield, Professor Carl V.
Thompson
Very small particles on a planar substrate can
exchange material by atomic diffusion of the particle constituent on the substrate surface. This
generally leads to an increase in the average particle size and spacing and can also lead to the
development of restricted crystallographic orientations. This process can be very important in the
early stages of the formation of a thin film. We
have developed a theory to describe the evolution
of particle sizes and orientations and are testing
this theory by experimentally characterizing particle
coarsening in model systems. We have shown
that Au particles on amorphous SiN membranes
annealed in air undergo a coarsening process
which is described well by the theory for interfacereaction-limited coarsening. We have also shown
that differences in the gaseous ambient strongly
affect the coarsening rate.
Sponsors
We have demonstrated that grain growth in polycrystalline films on single crystal substrates can
lead to epitaxial films. This new approach to
obtaining heteroepitaxial films can lead to ultrathin
films with reduced defect densities compared to
films deposited using conventional techniques. In
epitaxial grain growth, ultrathin polycrystalline
films are deposited on single crystal substrates.
When these polycrystalline films are heated to elevated temperatures, epitaxial grains with low filmsubstrate interface energies grow and consume
misoriented grains. Because the initial polycrystalline films are deposited at low temperatures,
fully continuous ultrathin films can be obtained.
Conventional Volmer-Weber epitaxy which is
carried out at higher deposition temperatures can
not be used to obtain equivalently thin epitaxial
films. We have developed kinetic analyses for
epitaxial grain growth and are testing these analyses through experiments on model systems,
1 Harvard-MIT Health Sciences Program, Cambridge, Massachusetts.
2 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire.
3 IBM Corporation, Thomas J. Watson Research Center, Yorktown Heights, New York.
Chapter 2. Microstructural Evolution in Thin Films
including Au and Ag films on
Experiments on epitaxial grain
viding a means of characterizing
interface energy, as a function
tation.
mica and NaCI.
growth are prothe film-substrate
of crystal orien-
2.4 Properties of Grain
Boundaries in Zone Melted
Silicon Thin Films
Sponsor
IBM Corporation
2.3 Modeling of
Microstructural Evolution in
Thin Films
Project Staff
Dr. Paul Evans, David A. Smith, Professor Carl V.
Thompson
Sponsors
Joint Services Electronics Program
Contract DAAL03-89-C-0001
National Science Foundation
U.S. Air Force - Office of Scientific Research
Project Staff
Professor Carl V. Thompson,
Jerrold A. Floro
Harold J.
Frost,
We are developing analytic models for normal and
secondary grain growth in continuous thin films as
well as particle coarsening in discontinuous films.
The effects of surface or interface energy
anisotropy play especially important roles in these
processes. We have developed computer models
for film formation by crystal nucleation and growth
to impingement under a variety of conditions. We
have shown that topology and geometry of grain
structures strongly depend on the conditions of
film formation.
We have also developed a computer model for
two-dimensional grain growth. This simulation
has been modified to account for the important
effects that the surfaces of a film have on grain
growth. We have shown that when formation of
grain boundary surface grooves leads to stagnation
of normal grain growth, lognormal grain size distributions with average grain size two to three
times the film thickness result. This is in agreement with well-established experimental observaWe have also successfully modeled
tions.
abnormal grain growth caused by anisotropy of the
surface energy of grains.
We are using zone melting recrystallization (ZMR)
of thin silicon films on oxidized silicon wafers to
prepare thin film bicrystals in order to study the
electronic properties of grain boundaries in silicon.
We are correlating electronic properties with structural features, as revealed using high resolution
electron microscopy. We have found that in these
samples, (100) tilt boundaries with tilts up to 25.5
degrees are electrically inactive. This surprising
result indicates that even polycrystalline ZMR films
might be useful for silicon-on-insulator (SOI)
majority carrier devices.
2.5 Kinetics of Thin Film
Silicide Formation
Sponsors
Hitachi Corporation
IBM Corporation
Project Staff
Professor Carl V. Thompson, Dr. En Ma, Dr. H.
Miura, King-N. Tu
Currently, there is considerable interest in the use
of refractory metals or refractory metal silicides as
interconnects, as gate materials in MOS devices
and for low contact resistance diffusion barriers at
metal-silicon contacts in integrated circuits. One
method of silicide formation is through the
reaction of metallic thin films with silicon substrates or polycrystalline silicon films. This application raises fundamental questions about the rate
and products of thin film metal-silicon reactions.
There are four critical parameters in analysis and
modeling of these reactions: (1) interdiffusivities,
(2) free energy changes, (3) surface energies, and
(4) interface reaction constants. Of these, the first
two parameters are fairly well understood and predictable. The purpose of this project is to develop
a better understanding and predictive capability for
Surface energies are
the last two parameters.
being determined through silicide precipitation
experiments and the kinetics of thin film reactions
24
RLE Progress Report Number 133
Chapter 2. Microstructural Evolution in Thin Films
are being studied through thermal, TEM, and x-ray
analysis of reactions in multilayer thin films.
We have used differential scanning calorimetry,
transmission electron microscopy, electron beam
microanalysis, and thin film x-ray diffractometry to
study the thermodynamics and kinetics of reactions in multilayer thin films. In Pt/amorphous-Si
(Pt/a-Si), Ni/a-Si, V/a-Si and Ti/a-Si multilayers,
amorphous silicides are the first phases to form,
even though these phases are thermodynamically
stable only if crystalline silicide formation is
kinetically suppressed.
We have demonstrated that calorimetric analysis of
multilayer films can be used to detect and analyze
crystalline silicide nucleation during reactions in
multilayers. Evidence for crystal nucleation has
been observed in the cases listed above as well as
in Co/a-Si and Ni/Al multilayers in which crystalline phases are the first phases to form. We have
accumulated experimental and theoretical evidence
which suggests that nucleation is preceded by
interdiffusion and that it is the kinetic constraints
of these sequential processes which govern phase
selection during interfacial reactions.
We have also observed explosive reactions in
multilayer metal/a-Si films and fully crystalline
Ni/Al multilayers. These reactions can propagate
in a room temperature ambient at velocities over
20 meters per second. This self-rapid-thermalannealing process results in homogeneous films
composed of the stable high temperature product
phase.
We are also investigating techniques for controlling microstructures in order to improve contact
and interconnect reliability, especially under conditions which can lead to electromigration.
We have recently shown that in interconnect lines
with uniform microstructures, increasing the grain
size results in an increase of both the median time
to electromigration-induced
failure and the
lognormal standard deviation in the time to failure.
The net result, in large populations of lines, is little
or no change in the time to the first failure. We
have explained these results in terms of a "failure
unit model" in which grain boundaries are taken to
be the individual units which are responsible for
the reliability of a line. The successful application
of this model indicates the importance of the properties of individual grain boundaries in controlling
interconnect reliability.
This interpretation is further supported by the
observation that interconnects with bimodal grain
size distributions (leading to grain size discontinuities) have greatly reduced reliabilities. On the
other hand, lines which are completely free of
grain boundary triple junctions have greatly
improved reliabilities compared to lines with comparable grain sizes but also with triple junctions.
We are now investigating the development of
electromigration-induced damage in lines with
individual grain boundaries of controlled types and
locations. This will allow characterization of the
failure mechanisms and rates for the fundamental
units that control the reliability of interconnect
systems.
2.6 Reliability and
Microstructures of
Interconnects
2.7 Focused Ion Beam Induced
Deposition
Sponsors
IBM Corporation
Joint Services Electronics Program
Contract DAAL03-89-C-0001
Semiconductor Research Corporation
Project Staff
Project Staff
Jaeshin Cho, Harold Kahn, Hai P. Lon gworth, Professor Carl V. Thompson
We are developing new techniques which allow
statistical characterization of failure of contact vias
and interconnects for integrated circuits. We are
using these techniques to correlate failure rates
and mechanisms with microstructures of interconnect lines, contact diffusion barriers and via plugs.
Sponsor
Dr. John Melngailis, Andrew D. Dubner, Jaesang
Ro, Professor Carl V. Thompson
It is now possible to produce ion beams with
diameters as small as 500 A. This permits use of
focused ion beams for high spatial resolution
implantation, sputtering and deposition. In principal, the latter can be used in integrated circuit
mask repair or high resolution direct writing of
interconnects. We are investigating the mechanisms of ion-beam-induced chemical vapor deposition from metal-bearing gases.
Chapter 2. Microstructural Evolution in Thin Films
2.8 Protective Coatings for
Integrated Circuits in an in
vitro Environment
Sponsor
National Institutes of Health
Project Staff
David J. Edell, Professor Carl V. Thompson
We are investigating the use of various coating
materials to prevent Na diffusion into integrated
circuits to be used in biomedical applications. We
are correlating processing conditions, microstructural characteristics and diffusion barrier properties
to develop standard methodologies for deposition
and characterization of protective coatings.
2.9 Publications
Cammarata, R.C., C.V. Thompson, C. Haydelden,
and K.N. Tu. "Silicide Precipitation and Silicon
Crystallization in Nickel Implanted Amorphous
Silicon Thin Films." J. Mater. Res. 5: 2133
(1990).
Cho, J., and C.V. Thompson. "ElectromigrationInduced Failures in Interconnects with Bimodal
Grain Size Distributions." J. Electron. Mater.
19: 1207 (1990).
Clevenger, L.A., and C.V. Thompson. "Explosive
Silicidation in Nickel/Amorphous-Silicon Multilayer Thin Films." J. App. Phys. 67: 2894
(1990).
Multilayer Thin Films." J. Vac. Sci. Tech. A 8
(3): 1566 (1990).
De Avillez, R.R., L.A. Clevenger, C.V. Thompson,
"Quantitative Investigation of
and K-N. Tu.
Titanium/Amorphous-Silicon Multilayer Thin
Film Reactions." J. Mater. Res. 4: 593 (1990).
Frost, H.J., C.V. Thompson, and D.T. Walton.
"Simulation of Thin Film Grain Structures: I.
Grain Growth Stagnation," Acta Metall. Mater.
38: 1455 (1990).
Jiran, E., and C.V. Thompson. "Capillary Instabilities in Thin Films." J. Electron. Mater. 19: 1155
(1990).
Kim, H.-J., and C.V. Thompson. "The Effects of
Dopants on Surface- Energy- Driven Secondary
Grain Growth in Silicon Films." J. Appl. Phys.
67: 757 (1990).
Ma, E., and C.V. Thompson. "Self-Propagating
Explosive Reactions in AI/Ni Multilayer Thin
Films." Appi. Phys. Lett. B7: 1262 (1990).
Thompson, C.V. "Grain Growth in Thin Films."
Ann. Rev. Mater. Sci. 20: 245-68 (1990).
Smith.
Thompson, C.V., J. Floro, and H.I.
"Epitaxial Grain Growth in Thin Metal Films."
J. Appl. Phys. 67 (9): 4-99 (1990).
Theses
Cho, J. Effect of Microstructure of Aluminum
Alloys on the Electromigration-Limited Reliability of VLSI Interconnects, Ph.D. diss. Dept.
of Mater. Sci. and Eng., MIT, 1990.
Clevenger, L.A., and C.V. Thompson. "Nucleation
Limited Phase Selection During Reactions in
Nickel Amorphous-Silicon Multilayer Thin
Films." J. Appi. Phys. 67: 1325 (1990).
Dubner, A.D. Mechanisms of Ion Beam Induced
Deposition. Ph.D. diss. Dept. of Mater. Sci.
and Eng., MIT, 1990.
Clevenger, L.A., C.V. Thompson, R.R. De Avillez,
and E. Ma. "Nucleation Controlled Phase
Vanadium/Amorphous-Silicon
in
Selection
Jiran, E. Capillary Instabilities in Thin, Solid Films.
Ph.D. diss. Dept. of Mater. Sci. and Eng., MIT,
1990.
26
RLE Progress Report Number 133